1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device in which a background of a liquid crystal display is achromatic.
2. Description of the Related Art
Liquid crystal display devices are classified into a TN type, a DS dynamic scattering type, a GH type, a DAP deformation of aligned phases type, a thermal write type, and the like in accordance with operation modes. Of these types, a TN liquid crystal display device has been widely used as a display device of a portable calculator, a measuring instrument, or the like.
The TN liquid crystal display device has, however, problems of, e.g., insufficient contrast and a narrow viewing range. As a large display capacity or a large display area of a personal computer has been increasingly required, a demand has arisen for development of a liquid crystal display device having a new operation mode.
As a liquid crystal display device having a new operation mode, Japanese Patent Disclosure (Kokai) No. 60-10702 discloses an SBE (supertwisted birefringence effect), electrically controlled birefringence liquid crystal display device. In this SBE liquid crystal display device, first and second transparent substrates, each having a transparent electrode on at least its one surface, are arranged to oppose each other and sealed to form a cell, and a nematic liquid crystal is sealed in this cell. A distance between the opposing substrates is about 3 to 12 .mu.m.
Examples of the nematic liquid crystal are cyclohexane-, ester-, biphenyl-, and pyrimidine-based liquid crystals. A chiral agent is added to this nematic liquid crystal. Molecular axes of liquid crystal molecules are arranged in a twisted mode through 180.degree. to 360.degree. between the two transparent substrates. In this liquid crystal molecule, a molecule axis has a tilt angle .theta. larger than 5.degree. with respect to the substrate surface because of the presence of an alignment film on the substrate. A retardation value R (R=.DELTA.n.multidot.d.multidot.cos.sup.2 .theta.) of the liquid crystal cell is 0.6 to 1.4 .mu.m. In the above definition of R, .DELTA.n is the optical anisotropy of the liquid crystal composition, d is the cell thickness (substrate spacing) of the liquid crystal cell, and .theta. is the tilt angle.
In an SBE liquid crystal display device having a molecular axis twist angle of 270.degree., a first polarizer is arranged on a first substrate, and a second polarizer is arranged on a second substrate. In this case, most preferably, a transmission axis of the first polarizer is twisted through about 30.degree. clockwise with respect to a molecule alignment direction of the first substrate, and a transmission axis of the second polarizer is twisted through about 30.degree. counterclockwise or about 60.degree. clockwise with respect to the alignment direction of the second substrate.
In an SBE liquid crystal display device in which the transmission axis of the first polarizer is twisted through about 30.degree. clockwise with respect to the molecule alignment direction of the first substrate and the transmission axis of the second polarizer is twisted through about 30.degree. counterclockwise with respect to the alignment direction of the second substrate, a bright yellow display is obtained when no voltage is applied, and a black display is obtained when a voltage is applied (yellow mode).
In an SBE liquid crystal display device in which the transmission axis of the first polarizer is twisted through about 30.degree. clockwise with respect to the molecule alignment direction of the first substrate and the transmission axis of the second polarizer is twisted through about 60.degree. clockwise with respect to the alignment direction of the second substrate, a deep blue display is obtained when no voltage is applied, and a white display is obtained when a voltage is applied (blue mode).
In such an SBE liquid crystal display device, a change in transmitted light quickly responds to the voltage. Therefore, even in case of a display with a large line number operated, a high contrast and a wide viewing angle can be obtained.
As an example of a liquid crystal display device in which the tilt angle (.theta.) is decreased by a rubbing technique, an ST (supertwisted) liquid crystal display device having a liquid crystal twist angle of 100.degree. to 200.degree. is known (SID'86DIGET, P. 122).
FIGS. 5A and 5B are views for explaining a principle of a conventional SBE or ST liquid crystal display device which performs a display by a birefringence effect. Referring to FIG. 5A, a liquid crystal cell 5 is constituted by first and second substrates 1a and 1b and a liquid crystal composition sealed therebetween. A first polarizer 3 is arranged on the first substrate 1a, and a second polarizer 4 is arranged on the second substrate 1b.
As shown in FIG. 5B, linearly polarized light 103 transmitted through the first polarizer 3 is transmitted through the liquid crystal cell 5 and generally becomes elliptically polarized light 101'. The elliptically polarized light transmitted through the liquid crystal cell 5 is transmitted through the second polarizer 4 arranged at a predetermined angle, and is sensed by a human eye. At this time, the shape of an ellipse depends on a twist angle .PSI. which is the twist angle of a liquid crystal molecule of the liquid crystal cell 5, a retardation value, and a wavelength .lambda..
In general, since the transmission changes in accordance with the wavelength, transmitted light becomes chromatic. When a voltage is applied to the liquid crystal cell to change alignment of the liquid crystal molecules, the optical anisotropy .DELTA.n effectively changes, and the retardation value and transmission change accordingly. A liquid crystal display is performed by using such a principle.
As another example, Japanese Patent Disclosure (Kokai) No. 60-73525 discloses a liquid crystal display device in which the retardation value is 0.5 to 0.8 .mu.m, a liquid crystal cell having a liquid crystal molecule twist angle of 270.degree. is used, an optical axis of a polarizer arranged on each substrate is set at substantially 90.degree., and the optical axis of the polarizer set in a direction at which the twist angle of the liquid crystal is divided into two.
In any of the above conventional liquid crystal display devices, however, a background of a liquid crystal display becomes chromatic. For this reason, on a display using the conventional liquid crystal display device, black is displayed in a yellow background, and white is displayed in a blue background. Therefore, a readability evaluation differs in accordance with a visual sense of an observer, i.e., some observers evaluated that readability (e.g., a contrast) was degraded due to the background color. A color change caused by a viewing angle direction or temperature change is large.
In the TN liquid crystal display device, since a background of a liquid crystal display can be achromatic, a color display can be easily obtained by using a color filter. As described above, however, the TN liquid crystal display device has problems of insufficient contrast and viewing angle.
In the SBE liquid crystal display device, although a high contrast and a wide viewing angle can be obtained, a color display cannot be obtained because a background is chromatic. That is, it is difficult to manufacture a liquid crystal display device in which a high contrast and a wide viewing angle can be obtained and a background of a liquid crystal display is achromatic.
An OMI liquid crystal display device is known as a device which solves the above problems (Appl. Phys. Lett. 50(5), 1987, P. 236). This OMI liquid crystal display device has a liquid crystal twist angle of 180.degree. and a retardation value of 0.5 to 0.6 .mu.m. A transmission axis of one of the polarizers is parallel to a rubbing axis, and an angle defined between absorbing axes of the two polarizers is set to be 90.degree. with respect to the rubbing axis.
In the OMI liquid crystal display device, however, since a twist angle of liquid crystal molecules is 180.degree., a change in transmitted light with respect to a voltage is not so rapid. Therefore, when a drive duty ratio is decreased, problems such as an insufficient contrast ratio, a narrow viewing angle, and a dark background arise.
Japanese Patent Disclosure (Kokai) Nos. 57-46227, 57-96315, and 57-125919 disclose two-layer-cell liquid crystal display devices which ca solve the dark background and the insufficient contrast. These devices include two TN liquid crystal cells arranged one upon the other, and polarizers arranged on both sides of the cell structure, and accomplish B/W display.
JJAP (26, NOV. 11. L177 84 (1987)) describes an example in which the above technique is applied to the SBE liquid crystal display device.
This two-layer-cell liquid crystal display device will be described with reference to FIGS. 6A and 6B. Referring to FIG. 6A, first and second liquid crystal cells 5 and 6 each having the above arrangement overlap each other. The two liquid crystal cells overlap such that twist directions of liquid crystal compositions of the respective cells oppose each other, and retardation values of the liquid crystal cells are substantially equal to each other. A first polarizer plate 3 is arranged on a substrate 1a of the first cell 5, and a second polarizer 4 is arranged on the substrate 2b of the second cell 6.
As shown in FIG. 6B, linearly polarized light 103 transmitted through the first polarizer 3 is transmitted through the first cell 5 to become elliptically polarized light 101'. This elliptically polarized light is transmitted through the second cell 6 to become linearly polarized light 102', transmitted through the second polarizer 4, and then sensed by a human eye.
In this case, it is important that the first and second cells 5 and 6 optically complement each other. Although the liquid crystal cells need not be complete optical complements of each other, an error between the retardation values of the liquid crystal cells preferably falls within the range of .+-.0.05 .mu.m. As a result, wavelength-dependence of the elliptically polarized light transmitted through the first cell 5 becomes complementary with wavelength-dependence of the elliptically polarized light transmitted through the second cell 6. That is, the light transmitted through the first and second cells 5 and 6 does not have wavelength-dependence. Therefore, the liquid crystal display device of this type has an achromaic background. This means that all light components in a visible range can be used in a display and a bright display can be obtained.
In the two-layer liquid crystal cell ST liquid crystal display device, electrodes are formed on inner surfaces of the opposing substrates 1a and 1b and driven as in a normal dot-matrix liquid crystal display device. However, no electrode is formed on the substrates 2a and 2b of the second cell 6 (i.e., the liquid crystal is not driven). That is, the second cell 6 is used to merely correct the elliptically polarized light.
As described above, the two-layer liquid crystal cell ST liquid crystal display device has advantages in that a b/w display can be obtained and the number of scanning lines can be increased. However, the two-layer liquid crystal cell ST liquid crystal display device has a narrower viewing angle than that of the SBE or OMI liquid crystal display device and is expensive because two liquid crystal cells are used.
As described above, the ST or SBE liquid crystal display device having a twist angle of 180.degree. has a liquid crystal display with a chromatic background. Although the OMI liquid crystal display device has an achromatic background, a liquid crystal display device having a high contrast and a bright background cannot be obtained.
The liquid crystal display device using two ST liquid cells can provide a b/w display with an achromatic background and a high contrast but is expensive.